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SMT-Sept2017

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34 SMT Magazine • September 2017
Bake Out after Cleaning?
After cleaning of the reworked PCBs, the
boards may need to be baked out. The vast ma-
jority of assemblies cleaned in an aqueous pro-
cess, with or without cleaning materials beyond
water alone, are dried but not baked out. A
small percentage of printed circuit board assem-
blies seeing water in their cleaning process are
followed by an oven-type extended bake out. If
a bake is required, then a 2- to 4-hour bake at
105°C or an 8- to 24-hour bake at 65°C is ade-
quate for typical FR-4 type assemblies.
The need for a bake out is also a function of
the type of drying system found in your clean-
er. If your cleaner utilizes an airknife to blow off
the water from the assemblies, then your chanc-
es of requiring a bake process are increased, es-
pecially with multilayer boards. Airknife drying
systems are most commonly found as part of
inline cleaning systems. If your cleaner utilizes
a convection/radiant forced heat type technol-
ogy to dry the assemblies, then there's no need
for bake out.
The answer to the question of whether a
bake is required after rework and re-washing of
a PCB is "it depends."
SMT
References
1. IPC-1601 Printed Circuit Board Handling
and Storage Guidelines.
Bob Wettermann is the principal
of BEST Inc., a contract rework and
repair facility in Chicago.
DRYING BOARDS AFTER REWORK CLEANING—TO DO OR NOT TO DO?
An international team by Nagoya University
has developed a new way of controlling the do-
main structure of ferroelectric materials, which
could accelerate development of future electron-
ic and electro-mechanical devices.
"We grew lead zirconate
titanate films on dif-
ferent substrate types to induce different kinds of
physical strain, and then selectively etched parts
of the films to create nanorods," says lead author
Tomoaki Yamada.
"The domain structure of the
nanorods was almost completely flipped com-
pared with [that of] the thin film."
Lead zirconate titanate is a common type of
ferroelectric material, which switches based on
the movement of trapped lead atoms between
two stable positions in the crystal lattice. Parts of
the film were deliberately removed to leave free
-
standing rods on the substrates. The team then
used synchrotron X-ray radiation to probe the
domain structure of individual rods.
The contact area of the rods with the sub-
strate was greatly reduced and the domain prop-
erties were influenced more by the surrounding
environment, which mixed up the domain struc-
ture. The team found that coating the rods with
a metal could screen the effects of the air and
they tended to recover the original domain struc-
ture, as determined by the substrate.
"There are few effective ways of manipulating
the domain structure of ferroelectric materials,
and this becomes more difficult when the mate-
rial is nanostructured and the contact area with
the substrate is small," says collaborator Nava
Setter.
"We
have learned that it's possible to
nanostructure these materials with control over
their domains, which is an essential step towards
the new functional nanoscale devices promised
by these materials."
Nagoya-led Team Flips the Switch on Ferroelectrics